Writing big-endian values

All the data in an aco file is stored in big-endian format and therefore needs to be reversed on Windows systems before writing it back into the file.

We can use the following two methods to write a short or a int respectively into a stream as a series of bytes. Of course, if you just want functions to convert these into bytes you could use similar code, just remove the bit-shift.

Writing the file

When writing the file, I'm going to follow the specification's suggestion of writing a version 1 palette (for backwards compatibility), followed by a version 2 palette (for applications that support swatch names).

With that done, we loop through each colour, calculate the four values that comprise the colour data and then write that.

If it's a version 2 file, we also write the swatch name. As these basic examples are just using the Color class, there's no real flexibility in names, so we cheat - if it's a "named" colour, then we use the Color.Name property. Otherwise, we generate a Swatch <index> name.

Converting colour spaces

As previously mentioned, the specification states that each colour is comprised of four values. Even if a particular colour space doesn't use all four (for example Grayscale just uses one, you still need to write the other values, typically as zero's.

Although it's a slight duplication, I'll include the description table for colour spaces to allow easy reference of the value types.

Id

Description

0

RGB.
The first three values in the colour data are red, green, and blue. They are full unsigned 16-bit values as in Apple's RGBColordata structure. Pure red = 65535, 0, 0.

1

HSB.
The first three values in the colour data are hue, saturation, and brightness. They are full unsigned 16-bit values as in Apple's HSVColordata structure. Pure red = 0,65535, 65535.

2

CMYK.
The four values in the colour data are cyan, magenta, yellow, and black. They are full unsigned 16-bit values.
For example, pure cyan = 0,65535,65535,65535.

7

Lab.
The first three values in the colour data are lightness, a chrominance, and b chrominance.
Lightness is a 16-bit value from 0...10000. Chrominance components are each 16-bit values from -12800...12700. Gray values are represented by chrominance components of 0. Pure white = 10000,0,0.

8

Grayscale.
The first value in the colour data is the gray value, from 0...10000.

While supporting CMYK colours are beyond the scope of this article as they require colour profiles, and I haven't the foggiest on the Lab space, we can easily support RGB, HSL and Grayscale spaces.

RGB is the simplest as .NET colours are already in this format. The only thing we have to do is multiple each channel by 256 as the specification uses the range 0-65535 rather than the typical 0-255.

Notice value4 is simply initialized to zero as this space only needs 3 of the 4 values.

We can also support HSL without too much trouble as the Color class already includes methods for extracting these values. Again, we need to do a little fiddling to change the numbers into the range used by the specification.

The last format we can easily support is grayscale. If the source colour is already grey (i.e. the red, green and blue channels are all the same value), then we use that, otherwise we'll average the 3 channels and use that as the value.

Demo Application

The usual sample application is available from the links at the end of this article. The sample generates a random 256 colour palette, then writes this to a temporary file using the specified colour space. It then reads it back in, and displays both palettes side by side for comparison.

About The Author

The founder of Cyotek, Richard enjoys creating new blog content for the site. Much more though, he likes to develop programs, and can often found writing reams of code. A long term gamer, he has aspirations in one day creating an epic video game. Until that time, he is mostly content with adding new bugs to WebCopy and the other Cyotek products.

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